1. Field of the Invention
This invention is directed to an internal combustion engine electronic ignition spark advance system and, more particularly, to a position extrapolator for use in such a spark advance system which affords cost and performance advantages over prior art devices.
2. Description of the Prior Art
It is common knowledge in the internal combustion engine art that the timing of the firing of the spark plugs in an internal combustion engine should be changed as a function of engine speed and other engine parameters to obtain optimum performance from the engine. Generally, it is preferable to advance the timing as a function of engine RPM with spark plug firing occurring near top dead center of the piston stroke cycle at low engine RPM and at progressively earlier points in the piston stroke cycle as engine RPM increases.
Historically, regulation of the point in the cycle where spark occurs (spark advance) has been accomplished automatically by mechanical means dependent upon engine speed, or related to engine air pressure or vacuum, and engine rotation. Such spark advance techniques are commonly referred to as vacuum advance and centrifugal advance. Mechanical techniques for advancing the spark timing of an internal combustion engine as a function of engine speed are described in the text Internal Combustion Engines Analysis and Practice by Edward F. Obert, Second Edition 1950, (Chapter 14, Spark Ignition Engines).
In recent years, the conventional spark ignition system, including the conventional coil, breaker point and rotary distributor, has been replaced to an increasing degree by more advanced and sophisticated electronic ignition systems. Improved engine performance may be derived from such advanced electronic ignition systems by, for example increasing the electrical voltage applied to the spark plugs and also controlling a pulse width in accordance with the engine parameters. However, the engine performance is inhibited by mechanical spark advance techniques which are not commensurate with the improved performance provided by such sophisticated electronic ignition systems. Accordingly, in recent years a concerted effort has been made in the applicable technology to develop a spark advance timing system which, on the one hand, provides a substantial increase in accuracy and performance commensurate with the sophistication of the electronic ignition systems, but which, on the other hand, is of relatively low cost, a factor of particular importance in the automotive industry.
With the advent of microprocessor computers, the actual calculation of the appropriate point in the stroke cycle or the corresponding crank shaft angular position at which each spark plug should be fired, has become sufficiently accurate and relatively inexpensive to be commensurate with previously-mentioned electronic ignition systems. However, one significant problem area still remains, namely, that of providing a relatively low cost means for accurately determining when the crank shaft angular position corresponds to the desired position for spark ignition.
Usually, position detectors rely upon one or more sensors that produce a reference signal in the form of a pulse at a predetermined angular position of the crank shaft or of the distributor shaft of the engine. Thus, at least once during each rotation of the engine crank shaft or distributor shaft a signal is produced which accurately indicates the actual angular position of the shaft at that time. Furthermore, since the period of these pulses is inversely proportional to the rotational rate of the engine, this pulse period is often also used to provide a calculating device with an indication of engine RPM. Engine RPM is one of the engine parameters commonly used in calculating desired spark advance.
One prior art technique employed to determine the angular position of the crank shaft between reference pulses, is to provide a so called "toothed wheel", also rotated by the crank shaft or distributor shaft, to generate additional reference pulses at a substantially higher frequency corresponding for example to each 0.5.degree. of angular rotation. One such toothed wheel system is disclosed in U.S. Pat. No. 3,923,022, issued Dec. 2, 1975. The general concept of such a toothed wheel approach is described below in conjunction with prior art FIG. 1.
The resolution of the position determining device that employs a toothed wheel approach is proportional to the angular distance the individual teeth are spaced from one another, for example 0.5.degree.. The accuracy of the toothed wheel approach is usually not affected to any significant extent by engine acceleration or deceleration. However, the use of an additional sensor and multiple generating device, adds substantial additional cost to the overall ignition timing system. Thus, alternate techniques for determining angular position without incurring the cost associated with the toothed wheel approach are preferable, particularly if accuracy, especially during acceleration when engine performance is critical, can be kept commensurate with the sophistication of electronic ignition systems. Such alternate techniques fall generally into the category of devices called position extrapolators which in effect predict the crank shaft angular position, extrapolating information that is dependent on the periodicity of the comparatively low frequency sensing device.
One such extrapolation device in the prior art, and discussed below in conjuntion with prior art FIG. 2 employs extrapolation based upon a linear counting device that is reset by a reference pulse. Although such devices avoid the substantial cost penalties associated with the toothed wheel approach, they suffer from other disadvantages which, as will be seen below, are substantially alleviated by means of the present invention. One disadvantage associated with prior art position extrapolation devices is their inherent inaccuracy in an accelerating or decelerating engine, and another disadvantage relates to their special hardware requirements for a relatively broad range of engine speeds.
Accordingly, the present invention provides a position extrapolation device for an electronic ignition, spark timing system, that obviates the inherent cost penalties of prior art toothed wheel devices but which provides substantial improvement over prior art position extrapolation devices.
By way of example, high speed adders are needed to accomplish the frequency multiplication techniques utilized in the prior art. However, in order to attain the high speed performance associated with such prior art devices, it is usually necessary to use wider surface area gates because such wider areas gates permit the achievement of higher speeds by presenting lower resistance paths to current. Of course wider gates mean larger and more expensive chips even in metal oxide semiconductor large scale integration MOS/LSI. The requirement for high speed devices often dictates the use of parallel logic flow with an associated increase in number of elements required to handle paralled signals. This increase in the number of elements also contributes to an increase in chip size and cost. Furthermore, the linear counting techniques of prior art devices call for a relatively large number of bits to meet resolution requirements over a wide range of engine rates.